This invention relates to the manufacturing of cans and more particularly, to the application of a water soluble epoxy coating to the interior of a heated can.
Metal containers have long been used in the beverage industry as a convenient packaging means. Before a can is suitable for containment of a beverage it is necessary to coat the interior surface with a protective substance to prevent the contamination of the beverage with a metallic taste. This coating is generally applied by a single nozzle positioned off the can center and outside the can. A narrow fan of spray is directed across the can to impinge on a thin strip of the interior surface along the entire length of the sidewall and a portion of the bottom. The can is then rotated thereby depositing a coating of between about 1 and 3 ten-thousandths of an inch thick, when cured, upon the entire interior surface.
Recently a new design of a light-weight can has been developed and gained acceptance in the beverage industry. This can has an integral bottom which comprises an outer frustoconical surface extending downwardly and inwardly from the sidewalls, an annular bead for supporting the can, an inner frustoconical surface extending upwardly and inwardly from the annular supporting bead, and a recessed domed center panel extending inwardly and upwardly along the axis of the can from the inner frustoconical surface. The combination of the above elements in a number of different designs has allowed the use of thinner metal in the can, thereby reducing the cost, while preserving the resistance to eversion necessary in a container which will be subjected to pressures of up to 100 psi.
Because of the particular design configurations of the can bottoms, there are numerous difficulties in adequately coating them, and especially the inner frustoconical surfaces therein. For stability purposes the annular bead which supports the can has a radius of over 80% of the total can radius. The inner frustoconical surface begins at the annular bead and extends upwardly and inwardly at an angle of between 0 degrees and about 30 degrees with the longitudinal axis of the can. As this surface faces away from the can center and is near the sidewall, it is highly inaccessible to a coating material directed from outside the can. In fact, where a single spray nozzle is positioned off the can center and directed across the can, depending on the exact position of the nozzle and shape of the surface, it often is found that it is geometrically impossible to directly spray the inner frustoconical surfaces. At best, a very small area of this surface is in the direct path of the spray pattern.
U.S. Pat. No. 3,693,828 to Kneusel, et. al., teaches a method of spraying difficult to reach surfaces by employing two nozzle spray devices.
When cans are treated with a solvent based epoxy or when cans at room or ambient temperatures are treated with a water base epoxy, the inability to directly spray the inner frustoconical surfaces is not a serious drawback. Apparently a sufficient amount of coating material is deposited on this surface by mere deflection off the sidewall of the can. But where cans substantially above ambient temperature are being sprayed with a water soluble epoxy, difficulties are generally encountered in achieving an acceptable coating on the inner frustoconical surfaces.
Hot cans are commonly encountered because the interior of cans are treated directly after a decorative coating has been applied to the exterior surface and cured. Cans emerge from this curing step at temperatures in excess of 200 degrees F. Although some cooling occurs as the can is transported to the interior coating operation, it is not unusual for a can to enter the interior coating process at a temperature in excess of 160 degrees F. depending on the ambient temperature in the manufacturing plant.
For proper adhesion of water soluble epoxies to the can surface it is important that a relatively slow curing process take place. During the curing process the carrier is evaporated while the solid material is left in a near liquid state. Upon cooling, the solid material strongly adheres to the can surface.
When a can having a temperature above 140 degrees F. is encountered in the treating process flash evaporation of the carrier may occur resulting in improper curing and a poor bond between the coating material and the can surface. Surprisingly this does not prevent proper coating of the can surface in areas where the coating material is directly sprayed onto the can. Apparently flash evaporation only occurs to the initial droplets of material reaching the can thereby cooling the surface and leaving a sufficient deposit of carrier on the surface to allow proper curing. This is not true of the inner frustoconical surfaces which receive the coating material indirectly by deflection off the sidewalls. The combination of the coating being preheated by contact with the sidewall and the lessor amount of coating which is finally deposited on the frustoconical surface results in total evaporation of the carrier prior to curing rendering the coating defective.
Therefore, it is necessary to take corrective cooling action if a water soluble epoxy is to be used for the coating material. Cooling is usually accomplished by increasing the time between curing the exterior coating and treating the interior. This requires more trackwork to give the cans an extra 1 to 2 minute ride or an accumulator to store cans for 1 to 2 minutes. Both methods are undesirable in that space and trackwork are wasted and more importantly, an increase in the handling of the cans is required resulting in a greater risk of damage to the cans.